The disclosed subject matter relates generally to integrated circuit manufacturing and, more particularly, to a method and apparatus for optimizing an optical proximity correction model.
The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as photolithography, or simply lithography. As is well known, lithographic processes can be used to transfer a pattern of a reticle to a wafer.
For instance, patterns can be formed from a photoresist layer disposed on the wafer by passing light energy through a mask having an arrangement to image the desired pattern onto the photoresist layer. As a result, the pattern is transferred to the photoresist layer. In areas where the photoresist is sufficiently exposed, and after a development cycle, the photoresist material becomes soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, a hard mask layer, etc.). Portions of the photoresist layer not exposed to a threshold amount of light energy will not be removed and serve to protect the underlying layer during further processing of the wafer (e.g., etching exposed portions of the underlying layer, implanting ions into the wafer, etc.). Thereafter, the remaining portions of the photoresist layer can be removed.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. For example, feature size, line width, and the separation between features and lines are becoming increasingly smaller. In these sub-micron processes, yield is affected by factors such as mask pattern fidelity, optical proximity effects and photoresist processing. Some of the more prevalent concerns include line end pullback, corner rounding and line-width variations. These concerns are largely dependent on local pattern density and topology.
Optical proximity correction (OPC) has been used to improve image fidelity. In general, current OPC techniques involve running a computer simulation that takes an initial data set having information relating to the desired pattern and manipulates the data set to arrive at a corrected data set in an attempt to compensate for the above-mentioned concerns. A reticle can then be made in accordance with the corrected data set. Briefly, the OPC process can be governed by a set of geometrical rules (i.e., “rule-based OPC” employing fixed rules for geometric manipulation of the data set), a set of modeling principles (i.e., “model-based OPC” employing predetermined behavior data to drive geometric manipulation of the data set), or a hybrid combination of rule-based OPC and model-based OPC.
The process of generating an OPC model is time intensive and expensive. Techniques for evaluating OPC models involve intensively manual processes that are time consuming and prone to errors and/or omissions. Typically, OPC model building and validation is a one-time event that occurs well before products reach manufacturing. The model is validated based on test patterns when the process transfers to manufacturing. In generating and optimizing an OPC model, a library of test patterns is typically generated for use by a software tool. Due to constraints of the software OPC analysis tools, the test patterns are typically symmetric and the centered on a particular pattern (e.g., middle line of a group of lines). A typical OPC library includes thousands of candidate test patterns that may be used to train an OPC model.
A designer typically chooses a subset of the library entries as calibration sites for training the OPC model based on the expected lithography conditions. Another, smaller set of sites is defined for validating the model. The validation sites typically includes patterns that match actual features of a device design. Typically, the OPC model is initially trained using theoretical resist definitions based on the selected calibration sites. After the OPC model is trained using the calibration sites, one or more test wafers are printed using a reticle modified using the preliminary OPC model. The test wafers include features corresponding to at least a subset of the calibration and validation sites. Measurements from the test wafer are then used to further calibrate the model.
After establishing the calibrated OPC model, reticles are designed and fabricated for a device design. The reticle designs are modified in accordance with the OPC model such that the printed features more closely match the design requirements given the optical proximity effects present in the lithography process.
In the development of an OPC model, the selection of the calibration and validation sites plays an important role in determining the overall effectiveness of the model. A particular site has various characteristics that define it in terms of its optical properties. Commonly used optical properties include maximum intensity (IMAX), minimum intensity (IMIN), intensity slope, density factors, squares of the various properties, etc. It is difficult to correlate the optical parameters of the calibration sites to those of the validation sites to allow the construction of an OPC model that adequately covers the design space expected in the actual design.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.